Polyurethane lacquers which are stable to light and which are free from solvent or contain only a small amount of solvent require low viscosity, colorless or only slightly colored polyisocyanates with a high isocyanate content.
Low viscosity polyisocyanates which are formed by the polymerization of hexamethylene diisocyanate are potentially very suitable for this purpose. Hexamethylene diisocyanate is a product which is readily available commercially, and the polymerization thereof, to form a polyisocyanate containing isocyanurate groups has been known for a long time from the literature. However, the polymers of hexamethylene diisocyanate (hereinafter known as HDI) containing NCO groups, which are obtained by the known production processes, are unsuitable or are only suitable to a limited extent for use in the production of lacquers which are stable to light and which are free from solvent or contain only a small amount of solvent.
Thus, for example, the HDI polymers which can be obtained in accordance with German Offenlegungsschrift No. 2,616,415 cannot meet all the requirements of lacquer polyisocyanates for high quality polyurethane lacquers, in spite of their relatively low viscosity, owing to their intrinsic color which leads to discoloration of the lacquer coatings. The same applies to the HDI polymer described in German Auslegeschrift No. 2,226,191, in column 5, line 55 to column 6, line 6. The last-mentioned HDI polymer also has a high content of dimerized HDI. Owing to the splitting up of this dimeric HDI, more free HDI is formed very easily during storage of the polyisocyanate so that, after sometime, the product has a high content of free HDI.
Other known processes for the polymerization of HDI use metal compounds as catalysts, for example, alkali metal or lead salts. According to British Pat. No. 809,809, these catalysts have the advantage that they lead to products which contain little or no dimeric material. Catalysts with metal compounds does however have several disadvantages. Since this reaction takes place quite vigorously, high viscosity, in part inhomogeneous products with a low NCO content are formed (cf. U.S. Pat. No. 3,330,828--Example 6; British Pat. No. 925,931--Example 2; or German Auslegeschrift No. 1,013,869--Example 4).
Owing to the exothermal reaction which takes place inhomogeneously, particles of gel which can be removed with difficulty only after being diluted with solvent, are also produced during metal catalysis (British Pat. No. 966,338). Another substantial disadvantage of the metal compounds recommended as catalyst in the publications is that when the catalysis is stopped, inorganic salts are formed which are insoluble in the polyisocyanate and cause haze and can also only be removed after dilution with a large amount of solvent, if at all.
According to the process described in U.S. Pat. No. 3,211,703, the reaction product during the polymerization of HDI is also either a gel or occurs in a dilute solution and cannot therefore be used in lacquers which are free from solvent or low in solvent. U.S. Pat. No. 3,211,703 does not disclose how HDI could be trimerized to form a liquid, solvent-free polyisocyanate. The same applies to German Offenlegungsschrift No. 2,644,684. In that specification, the same catalyst system of an alkylene oxide and N,N'-endoethylene piperazine is recommended, in particular for the polymerization of cycloaliphatic isocyanates as in U.S. Pat. No. 3,211,703. Moreover, the teaching in German Offenlegungsschrift No. 2,644,684 does not go beyond the disclosure in U.S. Pat. No. 3,211,703.
In German Offenlegungsschrift No. 2,325,826, aziridine or an aziridine derivation in combination with a tertiary amine is recommended for the catalysis of the trimerization of isocyanate groups. Apart from the disadvantage of the simultaneous use of solvents, the process in the last-mentioned Offenlegungsschrift has the particular disadvantage that poisonous catalysts which are known to be carcinogenic have to be used. The long incubation period of the polymerization reaction is another disadvantage.
According to German Auslegeschrift No. 1,150,080, quaternary ammonium hydroxides are used as catalysts for the trimerization of isocyanates. As illustrated by the embodiments in this publication, the above-mentioned catalysts are in practice used only for the polymerization of aromatic isocyanates. Attempts to polymerize aliphatic isocyanates and, in particular, HDI in accordance with the teaching in that Auslegeschrift, in the absence of solvents lead to inhomogeneous reaction products which cannot be used as the isocyanate component in high quality polyurethane lacquers. The unsuitability of the catalysts recommended in the Auslegeschrift referred to for the trimerization of aliphatic isocyanates is also revealed by Example 13 in which the trimerization of n-hexadecyl isocyanate only leads to a yield of the corresponding trimerizate laying below 50%, even after a reaction time of 4 days.
It can be seen from the foregoing that no process which can be carried out commercially in a simple manner and which allows the problem-free preparation of virtually colorless, low-viscosity, solvent-free polyisocyanates, containing isocyanurate groups, based on HDI, has yet been disclosed. Accordingly, an object of the present invention was to provide such a process.
This object could surprisingly be accomplished in accordance with process of this invention by using ammonium hydroxides carrying hydroxyalkyl substituents at the nitrogen atom as catalyst for the trimerisation of the isocyanate groups. This possibility to solve the problem of the invention is surprising since it has been known from U.S. Pat. No. 3,487,080 that such special ammonium hydroxides are unsuitable catalysts for the trimerisation of toluylene diisocyanates (TDI) which are known to be much more reative than hexamethylene diisocyanate (HDI). Thus it is clear from table 3 of said U.S. Patent that i.e. .beta.-hydroxyethyl-trimethyl-ammonium hydroxide is no catalyst for the trimerisation of TDI since the gelation time is indicated to be more than 4320 minutes. According to U.S. Pat. No. 3,487,080 it is therefore absolutely necessary to use such catalysts together with a second catalyst component.